The present invention relates to mass spectrometers and the like and in particular to a detector suited for, but not limited to, the detection of large ionized molecules in a mass spectrometer.
Mass spectrometers are analytic instruments that may provide for the precise measurement of the mass of molecules. Generally, the molecules to be measured are given an electrical charge and then accelerated by an electrical field. The velocity of their acceleration will be generally proportional to the mass-to-charge ratio (m/z) and so for a given and known charge the mass may be precisely determined by a charge, velocity, or energy measurement.
One method of determining velocity is the use of a “sector” type analyzer, which bends the trajectories of the charged particles using a magnetic field. When the particles exit the magnetic field, the angle of their trajectories (and spatial separation at a measurement point) will be in proportion to m/z and may be measured by a series of spatially separated collectors.
An alternative detection system uses a “time of flight” analyzer in which relative velocities of different molecular species are deduced based on the time it takes them to reach a detector. Common detectors used for time of flight analysis include so-called “Faraday cups” which are conductive metal cups, which catch charged particles and are attached to sensitive electrical amplifiers and “dynode” or microchannel detectors, which provide an amplification of received charge through electron multiplier techniques.
Mass spectrometry is increasingly applied to extremely large molecules, for example, proteins, that may be ionized by various techniques such as matrix assisted laser desorption/ionization (MALDI) in which the fragile proteins are protected with a matrix material that is struck by a laser beam. The matrix absorbs the energy of the beam and is removed from the protein while transferring a charge to the protein.
The large mass of proteins and similar bio-molecules decreases the sensitivity of a time of flight spectrometer because the velocity of the proteins is lower and thus the difference between velocities of large masses is less. In addition, common microchannel detectors experience a decrease in secondary electron yield with increasing ion mass.
U.S. Pat. Nos. 8,274,059; 8,507,845; and 8,686,375, all assigned to the same assignee as the present invention and hereby incorporated by reference, teach detector systems for large ions that provide detector membranes that convert the kinetic energy of the impinging ions, at a front surface of the membrane, into electrons ejected from the rear surface of the membrane by field emission and/or secondary electron emission. The ejected electrons may be then detected by a microchannel plate. By combining the membrane in front of the microchannel plate, improved sensitivity to large molecules may be obtained.